Shielding of electron gun from vaporized getter by decomposable foil over electrode aperture



y 1965 R. E. BENWAY SHIELDING OF ELECTRON GUN FROM VAPORIZED GETTER BY DECOMPQSABLE FOIL OVER ELECTRODE APERTURE Filed Dec. 16. 1960 INVEN TOR. Rea-7r .6 BEA/WAY 7Z2; igtx Armin/7 United States Patent O SHIELDING F ELECTRQN GUN FRGM VAPOR- IZED GETTER BY DECGMPOSABLE FOIL OVER ELECTRODE APERTURE Robert E. Beuway, Marion, Ind, assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 16, 1960, Ser. No. 76,287 7 Claims. (Cl. 313-181) This invention relates to cathode ray tubes and particularly to a novel electron gun and to an improved method of gettering a cathode ray tube.

Cathode ray tubes of the type used in television receivers usually include an electron gun comprising a plurality of coaxial, apertured electrodes mounted in spaced relationship on one or more insulator support rods. The gun is disposed in a neck section of the tube envelope at one end thereof and is adapted to project a beam of electrons onto a phosphor screen which is disposed on a faceplate section at the opposite end of the envelope. A getter is usually mounted on the electrode of the gun nearest the phosphor screen. In processing the cathode ray tube, the getter is flashed or activated by radio frequency heating thereof by a high frequency coil placed externally around the neck of the tube. This results in the gettering material being vaporized and deposited in a large area film on internal surfaces of the tube.

Although the getter is usually designed and oriented to direct its flashed vapors away from the gun and toward the phosphor screen, some of the vapors nevertheless are forced back toward the gun and deposit on parts thereof. This getter deposit often causes stray emission from, and possibly voltage breakdown between, electrodes of the gun when operating voltages are applied thereto.

Stray emission is undesirable cold, or field, emission from elements of the electron gun other than the cathode. Electrons of stray emission are attracted, unfocused and unmodulated, to the phosphor screen and thus result in back lighting which causes low image contrast. In cases where the stray emission becomes severe enough, and particularly where getter vapors are deposited on the insulator support rods of the gun, arcing, or voltage breakdown, may occur. This arcing causes objectionable background noise in the tube operation, and in its severest form may actually cause injury to parts of the electron gun such as the cathode, thus resulting in impairment of electron emission.

The prior art has attempted to minimize undesirable deposit of getter vapors on the electron gun parts by providing the end electrode of the gun nearest the phosphor screen with a radial flange which extends outwardly very close to the wall of the envelope neck. Such a flange prevents passage of getter vapors down the neck between the neck wall and the electrodes of the gun. However, even where such a flange is provided, harmful amounts of getter vapors still pass down the tube neck through the central electron beam apertures of the electrodes of the gun.

It is therefore an object of this invention to provide a new and improved electron tube wherein getter vapors are substantially completely prevented from depositing on parts of the tube where they can cause stray emission and voltage breakdown.

It is another object of this invention to provide a novel electron gun structure wherein flashed getter vapors are prevented from passing through the electron beam apertures of the electrodes and depositing on parts of the gun.

It is also an object of this invention to provide a new and improved method of gettering an electron tube so that harmful deposit of getter vapors on parts of the electron gun is substantially completely avoided.

3,183,391 Patented May 11, 1965 Briefly, according to this invention a thin sheet of suitable material is mounted over the electron beam aperture of one of the gun electrodes between the getter and those surfaces of the gun where getter deposit can cause stray emission. This electrode is preferably the end electrode of the gun nearest the phosphor screen. The getter is flashed with this thin sheet in place. The thin sheet closes the aperture of the electrode in question and thus prevents getter vapors from passing therethrough and depositing on other of the gun parts. The thin sheet is of a material, preferably a metallic foil such as aluminum foil, which can be vaporized by bombardment with electrons. After flashing of the getter, the portion of the sheet overlying the aperture of the end electrode isburned away by directing an electron beam from the gun thereupon.

In the drawings:

FIG. 1 is a partial longitudinal section view of a cathode ray tube embodying the invention;

FIG. 2 is an enlarged detail sectional view of the elec tron gun of the tube of FIG. 1;

FIG. 3 is an enlarged detail sectional view of a part of the electron gun of FIG. 2 illustrating a modification of my invention.

In FIG. 1, an electron tube includes an envelope 12 comprising a neck section 14, a faceplate 16, and an interconnecting funnel section 18. An electron gun in the neck 14 is adapted to project an electron beam onto a phosphor screen 22 on the faceplate 16. The phosphor screen 22 may be of any suitable type, such as one com prising a layer of phosphor material on the faceplate 16 with a superimposed film of evaporated aluminum thereupon. A conductive coating 24 is provided on the internal surface of the funnel 18 and is connected to the phosphor screen 22 and to one of the electrodes of the electron gun 26 in a manner explained more fully hereinafter. A high voltage contact terminal, indicated schematically by the arrow 26, is provided for applying a suitable voltage to the coating 24 and to the electrode to which it is connected. The neck section 14 is closed at its free end with a stem structure 28, which includes a plurality of lead-in conductors 30 for applying suitable voltages to the electrodes to the electron gun 20.

In FIG. 2 the electron gun 20 of the tube 10 is shown in greater detail. It comprises a plurality of coaxial, tubular, centrally apertured electrodes including a control grid 32, a screen grid 34, a focusing electrode 36, and an accelerating electrode 38. These electrodes are mounted in coaxial spaced relationship along a pair of insulator, e.g., glass, rods by U-shaped mounting studs 41 which are fixed to the electrodes and embedded in the insulator rods. A tubular cathode 42 is mounted in a centrally apertured insulator disk 44 which is in turn fixed within the tubular control grid 32. The cathode 42 is provided with a closed end 46 which is coated with an electron emissive material.

In the operation of the tube 10, electrons are emitted from the cathode 42 and converged to a first crossover in the vicinity of the central aperture 48 of the screen grid 34. This crossover is then imaged on the phosphor screen 22 by an electron lens formed by the focusing and accelerating electrodes 36 and 38, respectively.

The accelerating electrode 38 is partially closed at one end by a cup-shaped flanged insert 50 which has a central aperture 52 through which the electron beam passes. The accelerating electrode 38 also has a radial flange 54, which extends outwardly to within a very short distance from the wall of the envelope neck 14.

A plurality of spring snubbers 56 are fixed to the flange 54 and bear outwardly against the conductive coating 24,

which extends slightly into the neck 14. The snubbers 56 serve both to provide an electrical connection to the accelerating electrode 38 and to support one end of the electron gun 20 centrally within the neck 14. The other end of the electron gun 20 is supported (not shown) on some of the lead-in conductors 30.

A getter 58 is mounted between the phosphor screen 22 and the accelerating electrode 38 on a support member 69 fixed to the flange 54. The getter 58 comprises a channel 62 in the form of a ring through which the electron beam can be projected. A quantity of suitable vaporizable gettering material 63 is contained in the channel 62. The channel 62 is open toward the phosphor screen 22 so as to direct flashed getter vapors principally in that direction. A thin sheet 64 of suitable material is fixed to the apertured cup-shaped insert 50 and overlies and closes the aperture 52 thereof. The sheet 64 may be secured to the cup 50 in any suitable manner such as by spot welding at a plurality of points around the aperture 52.

The material of the sheet 64 should be one which will not decompose when subjected to the bake-out processing commonly employed in the manufacture of cathode ray tubes. Usually this involves a heating of the entire tube to a temperature between 400 C. and 450 C. However, in some tube processing methods, a much lower temperature bake-out or even no bake out at all is used. In such cases the sheet 64 need not be capable of resisting decomposition at such elevated temperatures. The sheet 64 should also be both of a material and of sufficient thinness so that it can be vaporized by the electron beam of the gun on which it is incorporated. Also the vaporized deposit of the sheet material should be stable and not detrimental to the life of the tube in any way. The sheet should be electrically conductive so as to conduct away the bombarding electrons.

I have found aluminum foil to be entirely suitable. However, other metal foils can also be used, for example, titanium, zirconium, thorium, or Ceralloy foil.

In processing a tube such as the tube 10, when the getter is flashed, the gettering material thereof is rather violently vaporized- Although the getter flash is directed generally toward the phosphor screen 22, the vapors from the getter nevertheless are forced back against the electron gun of the tube. In order to reduce the amount of getter vapors which pass down the neck 14 and deposit on parts of the electron gun, the prior art has provided the radial flange 54 on the accelerating electrode 38. However,'even with the flange 54, in the absence of the sheet 64, getter vapors pass through the aperture 52 and deposit on the gun parts. Such deposit is particularly troublesome in an electron gun of the relatively short type herein described when it falls upon any one of the tubular electrodes 32, 34, 36,'or 38, or upon portions of the insulator support rods 40 between any two of these electrodes. As shown in FIG. 2, representative operating potentials for the electron gun 20 may be as follows:

Volts Cathode 42 Control grid 32 -50 Screen grid 34 400 Focus electrode 36 200 Accelerating electrode 38 20,000

With such voltages, at least a 200 volt difference of potential exists between any one of the four tubular electrodes and an adjacent electrode, and in one case a voltage diiference of 19,800 volts is present. Electric fields produced by such voltage differences are more than adequate for sustaining field emission from these electrodes. Also, the deposit of getter vapors, being of a somewhat conductive nature, acts to increase the electrical leakage between electrodes along the insulator supports 40, thus possibly resulting in complete voltage breakdown.

In accordance with this invention, the inclusion of the sheet 64 closing the electron beam aperture 52 of the end electrode 38, results in substantially complete elimination of flashed getter vapors passing into the inner regions of the electron gun 20 through the apertures of the electrodes. Getter vapors will be deposited on the end surface of the end electrode 38 facing the phosphor screen 22. However, this produces no harmful effect since this surface of the accelerating electrode 38 is in a free-field region, the accelerating electrode 38 being connected to the conductive coating 24.

In the electron gun 20 of FIG. 2, after the getter 58 is flashed and the getter material 63 vaporized and allowed to deposit on available surfaces of the tube, the electron gun 20 is energized to generate an electron beam which is projected onto that portion of the sheet 64 which overlies and closes the electron beam aperture 52. The sheet, being of a proper material and being sufficiently thin, is burned away to provide an aperture in the sheet 64 which is in register with the aperture 52. I have, for example, made the sheet 64 from aluminum foil having a thickness of 0.8 to 1.0 mil, and have burned this away with an electron beam of 1000 micro-amps and 20 kv. (voltage on accelerating electrode 38) in approximately two seconds.

In burning away the sheet 64 overlying the aperture 52,

it is preferred that the electron beam be defocused to a diameter equal to the aperture 52 at the sheet 64 and thus simultaneously bombard the entire area of the sheet overlying the aperture. If the beam is focused to a small spot onto the foil and scanned thereover, the sheet will be vaporized successively in spots. Such a removal of the sheet 64 may result in unvaporized pieces of the sheet being severed therefrom and falling free to remain as debris in the tube 10.

FIG. 3 illustrates a modification of my invention. In FIG. 3 a sheet of metallic foil 68 is mounted over the aperture 52 on the cathode side of the insert cup 50. In the FIG. 3 embodiment the metal foil 68, instead of being spot welded to the apertured insert 50, is sandwiched between the insert 50 and the tubular portion 70 of the accelerating electrode 38. The insert 50 may be welded to the tubular portion 70 or the radial flange 54 of the accelerating electrode 38.

By virtue of the sandwiched mounting of the foil in the FIG. 3 embodiment, uniformity of electrical contact between the foil 68 and the accelerating electrode 38 is provided completely around the aperture 52 rather than intermittently therearound as might result with a spot welding of the foil to the insert cup 50. The improved electrical contact as provided by the sandwiched structure of FIG. 3 results in improved circumferential uniformity of conduction of bombarding electrons into the insert 50 and thus improved uniformity and completeness of burning away of the foil. Thus, the possibility of leaving a slight amount of'ragged foil extending slightly into the aperture 52 around the perpihery thereof is reduced.

If the metal foil is mounted over an aperture of an electrode which is not a lens-forming aperture, the presence of a ragged burned-out edge may present no undesirable condition in any even. The aperture 52 of the accelerating electrode 38, not being a lens-forming aperture, provides an ideal place for mounting of the foil. On the other hand, such is not the case with either of the ends, or apertures, of the focusing or accelerating electrodes 36 or 38 adjacent each other, since it is between these two electrodes that, the focus lens is formed, To

permit a ragged edge at either of these apertures would introduce aberrations into the resulting focus lens. Thus, it is preferred that the foil be mounted across an electrode aperture other than a lens-forming aperture.

At a first consideration of the invention, it might appear that the vaporization of the foil shield 64 or 68 would itself produce vapors which would pass down the center of the electron gun 20 and there deposit to produce the same type of harmful deposits as result from getter vapors in prior art tubes. Although it is not fully understood why such harmful deposits by vaporization of the foil shields 64 or 68 do not occur, the fact remains that they do not.

The invention has been described as embodied in one specific type of very short electron gun. However, incorporation of this invention is in no Way limited to this type of gun nor to any one particular electrode such as the end electrode closest to the phosphor screen. In some cases it may actually be desired that more than one of the apertures of the electron gun he closed with foil shields. volving the overlying of an electrode aperture with foil, the term aperture is used in a broad sense. For example, the term aperture is meant to describe an unconstricted end opening in a tubular electrode as well as a constricting opening in a transverse wall of an electrode such as the insert 50. Thus, were it desired to provide the gun 20 without the apertured insert 50, the foil, such as foil 64 or foil 68, could be mounted directly overlying the entire end opening of the tubular electrode 38.

The getter of the electron tube has been representatively shown as a ring getter mounted on the end electrode of the gun. However, other getter structures may be employed. The getter may be of a different type such as a straight bar or a circular cup, and may be mounted closer to or farther from the end electrode of the electron gun and may be of a type which is flashed either by application of RF energy or otherwise. Moreover, the getter may be supported from a part of the tube other than the electron gun.

I claim:

1. An electron gun comprising a plurality of aligned electrodes including an apertured end electrode, a metal foil overlying the aperture of said end electrode, and a getter disposed on the side of said metal foil opposite the other electrodes of said gun.

2. An electron gun comprising a plurality of aligned electrodes including a source of electrons and an apertured electrode, a sheet of metal overlying the aperture of said apertured electrode, and vaporizable gettering ma terial disposed on the opposite side of said sheet from said source of electrons.

3. An electron gun comprising a plurality of coaxial tubular electrodes mounted in spaced electrically insulated relation on an insulator support member, a getter mounted on an end one of said electrodes and disposed in spaced relation from the end thereof opposite the other electrodes, and a sheet of material decomposable by electron bombardment mounted transversely across said end electrode.

4. An electron tube comprising an envelope including a neck section; an electron gun within said neck section, said gun comprising a plurality of coaxial apertured electrodes, an end one of said apertured electrodes having a Likewise, in describing the invention as inradial flange extending outward to adjacent the Wall of said neck section, and a sheet of metal foil overlying and closing the aperture of said one electrode; and a getter material disposed on the side of the metal foil opposite the other of said electrodes.

5. An electron tube comprising an envelope; an electron gun within said envelope, said gun including a source of electrons at one end thereof and an apertured electrode at the other end thereof; and gettering material which when vaporized tends to travel through the aperture of said apertured electrode toward said source of electrons, said apertured electrode having a sheet metal shield closing said aperture to prevent said travel of said vaporized material, said shield having a hole burned therethrough in register with said aperture by bombardment with the electrons from the beam of said gun, said gettering material being deposited on portions of said tube on the side or said shield opposite said source.

6. An electron tube comprising an envelope having a neck section and a faceplate, a phosphor screen on said faceplate, an electron gun disposed in said neck section and adapted to project an electron beam onto said screen, said gun comprising a plurality of electrodes having coaxial apertures therein through which said beam can be projected, a getter disposed between said gun and said screen, and a sheet of vaporizable material mounted transversely on the end electrode of said gun closest to said screen of the aperture thereof, said vaporizable material being nondetrimental to the life of the tube.

7. An electron discharge device having an electrode assembly for producing an electron beam, one of said electrodes being apertured for the passage of said beam, a metal foil overlying said aperture and a getter disposed on the side of said foil opposite the electrode assembly, whereby said foil acts as a shield for said electrode assembly during gettering operation.

References Cited by the Examiner UNITED STATES PATENTS 2,128,581 8/38 Gardner 2925.l7 X 2,153,616 4/39 Diels 31625 X 2,154,368 4/39 Van Der Tuuk et a1. 316-4 X 2,267,714 12/41 Borries et al 250-49.5 X 2,774,645 12/56 Batchelor 3162 FOREIGN PATENTS 884,289 4/43 France.

ROBERT-SEGAL, Acting Primary Examiner. RALPH G. NILSON, GEORGE N. WESTBY, Examiners. 

1. AN ELECTRON GUN COMPRISING A PLURALITY OF ALIGNED ELECTRODES INCLUDING AN APERTURED END ELECTRODE, A METAL FOIL OVERLYING THE APERTURE OF SAID END ELECTRODE, AND A GETTER DISPOSED ON THE SIDE OF SAID METAL FOIL OPPOSITE THE OTHER ELECTRODES OF SAID GUN. 